Physically, local area networks (LAN) comprise a transmission medium and network devices that transmit through it. The transmission medium is typically coaxial or twisted-pair wiring. The network devices or nodes are the network cards of computer workstations that utilize the network cabling to communicate with each other. Dedicated network devices such as hubs, repeaters, bridges, switches, and routers are also used to manage or extend a given LAN or act as inter-networking devices.
One of the most common protocols for a LAN is termed carrier sense multiple access with collision detection (CSMA/CD). This protocol is sometimes generically, but incorrectly, referred to as Ethernet, which is a product of the XEROX Corporation. I.E.E.E. has promulgated standards for this protocol; IEEE 802.3 covers 1-persistent CSMA/CD access method and physical layer specification. The protocol comes in various implementations, 10Base(2) and (5) are 10 megabit per second (MBPS) networks using different gauges of coaxial cable (2 and 5) in a bus topology. 10Base(T) also operates at 10 MBPS but uses twisted-pair cabling in a star topology in which each node connects to a hub. Newer 100 MBPS implementations such as 100Base(T) are becoming more common with 1 GigaBPS devices in planning and testing.
A number of problems can arise at a LAN""s physical layer. In the case of twisted-pair or coaxial wiring, the electrical conductors may become frayed or broken. The shielding may be damaged, failing to protect the conductors from surrounding electromagnetic interference and changing the cable""s characteristic impedance. Moreover, the terminators at the end of the network cables in bus topologies or the terminators in the nodes at the ends of the links in star topologies may be poorly matched to the characteristic impedance of the network""s cables or non-existent. This produces signal reflections that can impair the operation of the network.
Another potential problem with a network is the fact that cabling may be too long. The IEEE 802.3 10Base(T) protocol, for example, limits the cable length to 200 meters with repeaters. This restriction is placed on networks because signals require a non-trivial time to propagate through the entire length of a CSMA/CD network relative to the data rate of the network. Network devices, however, must have some assurance that after they have been transmitting for some minimum time that a collision will not thereafter occur. Additionally, the end of each packet transmission must be allowed to propagate across the entire network before the next transmission may take place. If the cabling is long, the time allocated to this may begin to consume too much of the network""s potential bandwidth.
A number of techniques exist for validating a network at the physical layer. The most common approach is called time domain reflectometry (TDR). According to this technique, a predetermined signal, typically a step-function, is injected onto the network cabling. The TDR system will then listen for any returning echoes. Echoes arise from the signal passing through regions of the cable where the characteristic impedance changes. Based upon the amplitude of these reflections and the delays between the transmission of the signal onto the cable at the sending-end and the receipt of the reflection back at the sending-end, the location of the impedance change, a frayed portion of cable for example, may be located.
TDR has been used to determine the length of the network cable and thus whether it conforms to the relevant protocol. Prior to testing, the network""s terminators are removed and the conductors are shorted together or open circuited. The length of the cable may then be determined based on the time delay between when the TDR signal is placed on the network and when the open- or short-circuit reflection is detected at the sending-end.
The problem with known cable length detection methods is that they rely on the removal of the terminators or on a reflection producing device. The terminators, however, are necessary to the proper operation of the network. Thus, the cable length can only be determined on a non-operational network. This requirement is not unduly restrictive in the case of validating a newly installed network since the TDR analysis may be performed prior to the installation of the terminators or attachment of the nodes. This requirement, however, negates the periodic monitoring of an operational network and the diagnosis of a previously installed network that is exhibiting problems.
In the invention, the location of a properly configured terminator i.e., a terminator that has been configured to closely match the characteristic impedance of the network cabling, can be remotely detected by analyzing the network""s response to a predetermined signal for skin effects. In more detail, the terminator produces a signature that is detectable at the sending-end when predetermined signal, such as a current step function, is injected onto the network cabling. The magnitude of the voltage at the sending-end will slowly increase. This results from the skin effects and accumulated d.c. resistance across the length of the cable as the step function propagates down its length. After a time corresponding to twice the propagation time between the sending-end and the terminator, the voltage will undergo an inflection. After this inflection, the voltage asymptotically returns to the voltage level initially produced when the step function was generated.
According to the invention, a network analysis device is connected to the network to inject a predetermined signal, such as the step function, onto the network cabling. The voltage response of the cabling to this excitation is first digitally sampled and then analyzed in a system controller. The system controller reviews the sampled data for the signature of the terminator and then locates it by reference to the delay between when the signal was placed on the cable and the detection of the signature.
In specific embodiments, the terminator is detected by determining a change in the influence of skin effects on the response resulting from the step function, which is preferably encased in a packet-like transmission, for example, reaching the terminator. The change in skin effects is evidenced by an inflection point in the induced voltage on the network cabling.
In other aspects of the embodiments, the signature of the terminator is detected by calculating a first-order differential of the response as a function of delay from the generation of the predetermined signal and located where the first order differential indicates an inflection at a highest time delay from the predetermined signal
According to another aspect, the invention features a computer network analysis method. The signal responses of communications links of a computer network are detected and terminators identified on the communications links based on the signal responses. The acquired information is then displayed, including measures of lengths of the communications links of the computer network.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.